1. Field of the Invention
The present invention relates to systems for driving an elongated web member between a pair of rotatable reels, and more particularly to systems for driving an ink ribbon between opposite reels in an impact printer.
2. History of the Prior Art
It is known to provide impact printers in which the individual hammers of a hammer bank are employed to impact an ink ribbon against print paper supported by a platen to effect printing. The printing may be formatted in different ways such as in dot matrix fashion where the desired characters or other printed indicia are comprised of a series of point like marks made by the impacting hammers. The hammer bank may be reciprocated relative to the print paper to optimize usage of the hammers. At the same time the ink ribbon which is typically mounted by an opposite pair of rotating reels may be driven in opposite directions between the opposite ends thereof to equalize usage and wear thereof.
One example of an impact printer system of the type described is provided by U.S. Pat. No. 3,941,051, Barrus et al, PRINTER SYSTEM, issued Mar. 2, 1976 and assigned to the assignee of the present application. In the impact printer system of U.S. Pat. No. 3,941,051 the opposite ribbon reels are driven by a pair of AC motors, one of which serves as the leading or take-up motor. The trailing or supply motor is coupled to operate as a torquer which exerts constant torque. Because the diameter of the ribbon pack varies as the ribbon is driven from one reel to the other, the constant speed of the leading motor results in proportional variation in the speed of the ribbon. For example, the speed will typically double as the ribbon pack diameter is doubled for constant rotational speed of the leading motor. Similarly, the torquer motor causes ribbon tension to vary inversely with the diameter of the ribbon pack on the reel connected to the torquer motor. Ribbon tension also varies with variations in the line voltage. There is a further tension variation at the print positions caused by the fact that the torquer motor can be either at the leading or trailing end of the ribbon depending upon ribbon direction, and tension drop around the fixed guides for the ribbon can either add to or subtract from tension as determined by the torquer motor. Because the leading motor and the torquer motor are physically two different type devices, it is not feasible to interchange their role when the direction of ribbon drive is reversed.
Further problems may arise in cases where the hammer bank prints while moving in both directions relative to the print paper. For maximum duty cycle printing there is a narrow speed range that will allow printing on a new ribbon section for both directions. However, in the absence of precise speed control on the ribbon, dense printing such as "all black" produces horizontal shading when the ribbon travels in one direction at a speed below the required level. Further problems arise from the fact that it is difficult to provide a reliable fault sensor for the ribbon path which will indicate failure of the ribbon drive or ribbon.
Driving of the ribbon at other than a constant speed results in non-uniform wear which greatly shortens the useful life of the ribbon. Variations in ribbon tension detract from optimum guiding of the ribbon around the fixed guides that are preferably used. Excessive tension causes the edges of the ribbon to curl or fold over themselves at the guides. Too little tension often results in a tendency of the ribbon to fold over itself in the region where it drags against the print paper. A further disadvantage in the non-uniform ribbon speed of conventional ribbon drive systems relates to the inability to minimize print on overlapping parts of the ribbon as it makes each pass by the print head.
Accordingly, it would be desirable to provide a ribbon drive system which drives the ribbon at substantially constant speed independent of the varying ribbon packs on the rotating reels.
It would furthermore be desirable to provide a ribbon drive system which maintains substantially constant tension in the ribbon independent of varying ribbon packs on the reels.
It would furthermore be desirable to provide a ribbon drive system which does not require tachometers or similar means for sensing ribbon velocity or tension, which provides a fault signal in the event the ribbon breaks, is unsecured to the spool, or the drive is stalled, and which reverses the roles of the two different motors as ribbon direction is reversed so as to achieve substantially constant ribbon speed and tension and the other desirable characteristics when driving the ribbon in either direction.